1. Field of the Invention
This invention relates to printed circuit board assemblies, and more particularly, to the use of thin decoupling capacitors in conjunction with ball-grid array (BGA) packages.
2. Description of the Relevant Art
Decoupling capacitors are used in electronic circuits to filter out transients in their associated power distribution systems. Transients such as voltage spikes and momentary voltage drops may have adverse affects on various circuits, and may possibly cause erroneous operation. Transients may often times be filtered out of a power distribution system by the use of capacitors (this is often referred to as decoupling). When electrically connected between a power line and an electrical ground, a capacitor will tend to filter out many transients, as the voltage across a capacitor cannot change instantaneously.
The amount of capacitance necessary for effective elimination of power system transients can vary based on several different factors. One such factor is the frequency of operation of the circuits for which decoupling is to be provided. Circuits which operate at higher frequencies, such as some radio circuits and high-speed computer systems, may produce more transients than low frequency or DC circuits. Furthermore, some high frequency circuits may produce a number of harmonics or sub-harmonics, which may result in transients at different frequencies.
Another factor which may affect the amount of capacitance required for effective decoupling is the distance between the circuits and the decoupling capacitors. Various factors may affect the placement of decoupling capacitors on a printed circuit board (PCB). One such factor is the packaging used for integrated circuits (IC""s) mounted on the PCB. Some types of packaging, in conjunction with other PCB layout constraints, may result in decoupling capacitors being placed a significant distance from their associated circuits. As the distance between the decoupling capacitors and associated circuits increase, inductance from the circuit lines connecting the capacitors to the packaging (known as Equivalent Series Inductance, or ESL) may become a problem. ESL provides an inductive element to the impedance between a power line and electrical ground, and may thus reduce the effect of the capacitive impedance. Thus, capacitors of greater value (or a greater number of capacitors) may be required to overcome the effects of ESL.
The use of ball-grid array (BGA) integrated circuit packages may add further complications to the problem of providing decoupling capacitance. FIGS. 1A and 1B illustrate two possible BGA configurations. Each drawing illustrates a view of the BGA from the bottom side (i.e. the side that mounts to the PCB). In FIG. 1A, the BGA shown includes a plurality of electrical contacts in-a xe2x80x9cringxe2x80x9d arrangement near the peripheries of the package. A plurality of electrical contacts is also located in the center of the package, with an open space in between. Typically, electrical contacts for conveying power in this type of BGA are located on the inner portion of the ring, with ground contacts located in the center. FIG. 1B illustrates a similar BGA arrangement, minus the central group of electrical contacts. In this arrangement, power and ground contacts are typically located on the inner portion of the ring. Both types of BGA""s may be mounted to corresponding contact pads on a PCB. The plurality of contact pads to which a BGA is mounted is sometimes referred to as a xe2x80x9cfootprintxe2x80x9d.
Typically, decoupling capacitance is provided for BGA""s through the use of one or more surface mounted capacitors. If the decoupling capacitors are mounted on the same side of the PCB as the BGA, they will typically be placed some distance away from the power and ground pins of the BGA package. In such cases, a greater number of capacitors may be required to overcome the effects of ESL. As an alternative, ESL may be minimized by mounting the decoupling capacitors nearer the power and ground contacts of the BGA, but on the opposite side of the PCB. While this alternative may reduce the effects of ESL, it may add complexity to the assembly of the finished product. By placing capacitors on the opposite side of the PCB, extra soldering operations may be required. Furthermore, gluing operations to secure the capacitors to the PCB prior to soldering may be required as well. The extra manufacturing operations may result in a significant increase in the cost of the finished product, and increase the likelihood of defects during the assembly process.
The problems outlined above may in large part be solved by a printed circuit board assembly with improved bypass decoupling for BGA packages, in accordance with the present invention. In one embodiment, a capacitor may be interposed between a BGA package and a PCB within a perimeter of the contact pads that form a BGA footprint. The capacitor may have physical dimensions which allow a BGA package to be mounted such that there is no physical contact between the capacitor and the BGA.
In one embodiment, the capacitor used is an thin capacitor, with a thickness of no more than 0.5 millimeters. The small thickness of the capacitor may allow it to be easily interposed between a BGA and a PCB.
One embodiment of the thin capacitor has no leads. Terminals located on the capacitor package may be soldered directly to the appropriate contact pads on the PCB. Since the package is leadless, equivalent series inductance (ESL) may be minimized. In some cases, the minimization of ESL may result in the need for fewer capacitors to effectively decouple a BGA package.
In various embodiments, multiple capacitors may be embodied within a common package. For example, in one embodiment an ultra-thin capacitor package includes two capacitors which differ in capacitance value by a factor of 10. By using packages with multiple capacitors, power system transients occurring at different frequencies may be effectively filtered out.
Thus, in various embodiments, improved bypass decoupling for BGA packages may be accomplished by interposing thin capacitors (or capacitor packages) between a BGA and a PCB. By interposing capacitors between a BGA and the PCB, some manufacturing operations may be eliminated which may result in cost reductions to the finished product. The use of leadless capacitor packages may help minimize ESL and thus allow the use of fewer capacitors to effectively decouple a BGA. The use of capacitor packages with multiple capacitors may allow the effective filtering of transients occurring at different frequencies. The use of open space under a BGA may allow for more efficient utilization of circuit board area for other components.